Electrical contact brush



Patented Mar. 18, 1947 UNITED ortellini liliwii 2 ELECTRICAL CONTACT BRUSH Dimiter Ramadanofi, Cleveland, Ohio, assignor to National Carbon Company, Inc., a corporation of New York No Drawing. Application February 11, 1943, Serial No. 475,523

4 Claims. 1

The invention relates to electrical contact brushes, and more specifically to carbon brushes for use in electrical machinery required to operate under conditions such as exist in the atmosphere at high altitudes.

Well-made brushes of natural or artificial graphite or other commercial forms of carbon, or f c'ombinations of carbon with one or more metals such as copper and silver, are very durable when operated in sliding contact against copper commutator'bars or slip-rings at sea level and altitudes up to about 15,000 feet above sea level. At high altitudes, wearing away of the brushes is accelerated, and under the atmospheric conditions encountered at altitudes above 25,000 feet above sea level the rate of Wear of the brushes is extremely rapid. A standard high-quality *carbon brush, when operated against a copper commutator in the air at 30,000 feet, for example, wears almost as rapidly as if it were bearing against an abrasive wheel. In some instances, brushes have been Worn out in less than an hour.

The most suitable of the known brush composiitions may in some instances last several hours;

but at best the brushes can not be relied upon for long.

Modern aircraft use electrical motors, generators, and other electrical devices requiring contact brushes. Both military and civil aircraft are being designed to operate at great altitudes and there is an urgent demand for electrical contact brushes that will operate dependably over a useful life at such altitudes. The principal object of this invention is to meet this demand.

The reasons for the poor behavior of brushes at high altitudes are not altogether clear; but it seems certain that two important factors are the dryness and the low oxygen pressure of the air. A similar behavior has sometimes been observed, during periods of unusually dry winter weather, even on the earths surface in the cases of some brush compositions; but the problem has not been a serious one. By operating brushes in a test chamber in which the atmosphere can be controlled, it has been found that either a low moisture content or a low partial pressure of oxygen may cause rapid brush wear, and it has also been found that many expedients which overcome the trouble caused by dry air do not materially offset the ill efiects of a low partial pressure of oxygen. These observations suggest that an important factor in the satisfactory operation of a carbon brush at low altitudes is a film between the commutator or ring and the brush, which film is maintained by moisture and oxygen in the air.

Further support for this hypothesis, and a suggestion that other film-forming materials might be substituted for moisture and oxygen are found in the observation that new electrical apparatus, still containing traces of slightly volatile compounds in its insulation and paint, may not give trouble at high altitudes until after several weeks or months of use. This latter circumstance increases the hazard, because rapidbrush wear may start without warning after a considerable period of satisfactory operation. In an airplane several hours from its base the brushes may suddenly start to Wear at a rate which will destroy them in an hour.

One approach to the problem is to impregnate the brushes with one or more substances which Will establish and maintain a suitable film between the commutator or slip ring and the carbon brush. But the choice of an impregnant material is a difiicult one, for much more is involved than the mere provision of a film-forming material. The material must be effective in small amounts, because the interstitial storage space in a brush body is not very great. It must be a liquid or liquefiable by melting or by solution to make impregnation possible, and it must not be filterable by the outer pores of the carbon. It must not evaporate rapidly. It must not exude, for exudation may result not only in undue depletion of the impregnant but also in the formation of a surface layer which tends to pick up dirt and to interfere with the smooth operation of the brush in its holder; and the film which it supplies must have a low electrical resistance, must not pick up dirt nor decompose to deleterious products, and must be stable under a Wide variety of conditions of atmosphere, temperature, and of mechanical and electrical stresses.

I have discovered that the incorporation in a carbonaceous brush of a small proportion of material selected from the group consisting of chromic anhydride (CrOz) and salts thereof acts to prolong materially the useful life of the brush in sliding contact with copper under dry atmospheric conditions such as exist at elevations over 25,000 feet above sea level. The invention is based on this discovery.

The invention is an electrical contact brush composed principally of material selected from carbon and mixtures of carbon with metal, and containing a small amount between 0.05% and. 2.5% by weight of material selected from the group consisting of chromic anhydride (CrOs) normal chromates but also the polychromates such as the dichromates.

The carbon or mixture of carbon with metal may be any of the many known brush materials composed of natural or artificial graphite or other forms of carbon or mixtures thereof, with or without metal such as copper or silver. A preferred material is a mixture of copper and artificial graphite.

The chromic anhydride or salt preferably amounts to between 0.25% and 1.5%, by weight, of the brush, about 0.5% being a convenient and suitable proportion in a copper-graphite brush. The material may be incorporated in the brush by impregnating the latter with a solution of the chromic anhydride or salt, and subsequently removing the solvent by evaporation.

In a specific instance, copper-graphite brush compositions which when operated in air containing less than 0.005 grain of water per cubic foot of air, wore at rates exceeding one-quarter inch per hour, when impregnated with a water solution containing 5% ammonium chromate and dried at room temperature, leaving about 0.5% ammonium chromate by weight in each 4 brush, wore less than 0.001 inch per hour under the same operating conditions.

I claim:

1. Electrical contact brush comprising material selected from the group consisting of carbon and mixtures of carbon with metal, and a minor proportion of an impregnant material acting to prolong the life of uch brush when operating in sliding contact with copper in a dry atmosphere, said last-mentioned material being selected from the group consisting of chromic anhydride and soluble salts thereof.

2. Electrical contact brush as claimed in claim 1, wherein the material selected from the group consisting of chromic anhydride and salts thereof amounts to between 0.05% and 2.5% by weight of the brush.

3. Electrical contact brush as claimed in claim 1, wherein said last-mentioned material is ammonium chromate in a proportion amounting to between 0.1% and 1% of the weight of the brush.

4. Electrical contact brush comprising carbon, copper, and about 0.5% CrOs.

DIMITER RAMADAN CFF. 

